1. Field of the Invention
The present invention relates to magnetoresistive sensors used as proprioceptors or position sensors, angle sensors and so on.
2. Description of the Related Art
Known magnetoresistive (MR) elements include anisotropic magnetoresistive (AMR) elements based on anisotropic magnetoresistive phenomena and giant magnetoresistive (GMR) elements based on spin-dependent scattering phenomena of conduction electrons. U.S. Pat. No. 5,159,513 discloses a spin-valve element having high magnetic resistance effects at a low external magnetic field, as an example of the GMR elements.
Since noncontact potentiometers and magnetic sensors using MR elements, such as noncontact angle sensors and position sensors, operate by direct current flows or low frequency current flows, use of high pass filters is not allowed when the DC offset component (the component of the resistance not changing by the magnetic field) is removed from the output voltages of the MR elements. Herein, the output voltage of the MR element is represented by the following equation: EQU V=(R.sub.0 +.DELTA.R)i
wherein R.sub.0 is the invariable component not changing by the magnetic field, .DELTA.R is the variable component changing by the magnetic field, i is a current flow in the element, and thus the DC offset component is represented by R.sub.0 i. Removal of the DC offset component is essential for the signal processing, such as amplification, in the succeeding circuits.
As shown in FIG. 6, in a typical conventional circuit configuration, an MR element 1 and another MR element 2, which have opposite signals to each other of a change in resistance to a magnetic field, are connected in series, two terminals A and C are provided at both unconnected ends of the MR elements 1 and 2, and an intermediate terminal B is provided between the MR elements 1 and 2 so as to collect the differential output between the two MR elements 1 and 2. The output V.sub.1 =(R.sub.0 +.DELTA.R)i of the MR element 1 is measured between the terminals A and B, and the output V.sub.2 =(R.sub.0.DELTA.R)i of the MR element 2 is measured between the terminals B and C, and thus differential output 2.DELTA.Ri as the variable component is given by 2.DELTA.Ri=V.sub.1 -V.sub.2.
Another conventional configuration is shown in FIG. 7. A bridge circuit includes four MR elements 3, 4, 5 and 6, wherein the MR elements 3 and 5 have the same signal of a change in resistance, the MR elements 4 and 6 also have the same signal of a change in resistance, and the signal of the MR elements 3 and 5 and the signal of the MR elements 4 and 6 are opposite to each other. Terminal a, b, c and d are provided between the MR elements 3 and 6, between the MR elements 4 and 5, between the MR elements 3 and 4, and between the MR elements 5 and 6, respectively. A variable component can be detected from the differential output voltages between these terminals by canceling the invariable component.
FIG. 8 shows a concrete example of the conventional circuit configuration shown in FIG. 6. In FIG. 8, two rectangular patterns 7 and 8 formed of magnetic films composed of a Ni--Fe alloy (permalloy) are arranged perpendicular to each other, terminals D and E are provided at the ends of pattern 7 and pattern 8, respectively, and an intermediate terminal F is provided at the connection point of the patterns 7 and 8. Since the resistance of each of the AMR elements 7 and 8 depends on the angle .theta. between the current i and the magnetization M in the direction of the arrow M, the patterns 7 and 8 are arranged perpendicular to each other. The changes in resistances of the AMR elements 7 and 8 to the magnetization M are respectively represented by the equations: EQU R.sub.1 =R.sub.0 -.DELTA.R.multidot.sin.sup.2 (90-.theta.), and EQU R.sub.2 =R.sub.0 -.DELTA.R.multidot.sin.sup.2.theta.,
The variable component therefore is determined by the following equation (I): EQU R=R.sub.1 -R.sub.2 =.DELTA.R[sin.sup.2.theta.-sin.sup.2 (90-.theta.)]=-.DELTA.R.multidot.cos2.theta. (I)
wherein R.sub.1 is the resistance of the AMR element 7, R.sub.2 is the resistance of the AMR element 8, and R.sub.0 is the invariable component of the resistance of each of the AMR elements 7 and 8.
In a sensor having the circuit configuration shown in FIG. 8, when the magnetization directions of the AMR elements 7 and 8 are simultaneously rotated by rotating a magnet provided near the AMR elements 7 and 8 (such a rotation corresponds to the operation of a noncontact potentiometer), the AMR element 7 rotates, for example, in the direction causing increasing .theta., whereas the AMR element 8 rotates in the direction causing decreasing .theta.. The output phases of the AMR elements 7 and 8 are therefore antiparallel to each other.
It may be expected that if spin-valve elements showing high magnetoresistive effects at a low external magnetic field among GMR elements are used instead of the AMR elements 7 and 8, a high output can be obtained because of a larger variable component of the magnetic resistance, and thus a sensor with a high sensitivity can be fabricated; however, since the variable component in the spin-valve elements does not depend on the angle .theta. between the magnetization M and current, the spin-valve elements cannot be used in the circuit configuration as shown in FIG. 8.
The above-mentioned U.S. Pat. No. 5,159,513 discloses a magnetoresistive sensor as an example of magnetic sensors using spin-valve elements. As shown in FIG. 9, this magnetoresistive sensor 10 has a layered structure composed of a free ferromagnetic layer 12, a nonmagnetic layer 13, a pinned ferromagnetic layer 14, and an antiferromagnetic layer 15 which are deposited on a nonmagnetic substrate 11. The vector 16 of magnetization of the pinned ferromagnetic layer 14 is fixed by magnetic exchange coupling with the antiferromagnetic layer 15, and the vector 17 of magnetization of the free ferromagnetic layer 12 is perpendicular to the vector 16 of magnetization of the pinned ferromagnetic layer 14. The vector 17 of magnetization of the free ferromagnetic layer 12 is, however, not fixed, and thus rotates by an external magnetic field.
When a magnetic field h is applied to such a configuration, the vector 17 of magnetization of the free ferromagnetic layer 12 rotates as shown in dotted arrows in FIG. 9 in response to the vector of the magnetic field h. The angle between the vectors of. magnetization of the free ferromagnetic layer 12 and pinned ferromagnetic layer 14 therefore changes, and thus the variable component of the magnetic resistance changes. The magnetic field can therefore be detected by the change in the resistance. The resistance depends on the angle .phi. between the vectors of magnetization of the pinned ferromagnetic layer 14 and free ferromagnetic layer 12; that is, the resistance has a minimum at .phi.=0.degree., and has a maximum at .phi.=180.degree..
The present inventor has conceived that when a pair of magnetic sensors are provided on a substrate so that the pinned ferromagnetic layers 14 have the opposite (antiparallel) vectors of magnetization from each other, these magnetic sensors will output reversed-phase signals from each other even when the vectors of magnetization of the free and pinned ferromagnetic layers rotate in the same direction, and thus a noncontact potentiometer, such as an angle sensor or position sensor, can be produced by using spin-valve elements.
In known spin-valve elements, however, the vectors of magnetization of the pinned ferromagnetic layers 14 are fixed by a unidirectional anisotropy by means of the antiferromagnetic layers 15, hence the vectors of magnetization of the pinned ferromagnetic layers 14 must be determined by depositing or annealing of the layers in a magnetic field. As a result, it is substantially impossible to provide pinned ferromagnetic layers having different vectors of magnetization in the two magnetic sensors on the substrate.
In prior art technologies for magnetic sensor fabrication, therefore, two spin-valve elements having reversed phases from each other have been formed on different wafers, and these two reverse-phase spin-valve elements have been arranged next to each other. Such a fabrication process causes high material and production costs. Further, these two spin-valve elements will have slightly different resistances because of the use of different wafers. Such different resistances prohibit the use of the spin-valve elements in the bridge configuration shown in FIG. 7.
A proposed configuration involves shielding with a magnetic film of an MR element 2 or 1 in the circuit shown in FIG. 6 or MR elements 3 and 5 (or 4 and 6) in the circuit shown in FIG. 7 from the external magnetic field, hence these elements functions as mere resistances. Although such a configuration can remove DC offset components, the output will decrease to half.
In a feasible production process of the spin-valve elements, a conductive pattern for generating a magnetic field during annealing is formed, and the vectors of the pinned ferromagnetic layers are controlled by annealing the spin-valve elements by conducting an electrical current in the conductive pattern. This process, however, requires a photolithographic step for forming the conductive pattern, and thus causes high production costs.